Introduction to JavaScript Drag and Drop

Support for drag and drop operations is implemented in JavaScript and may be applied to individual elements of your HTML page. For drag operations, an element can handle the following JavaScript events:

ondragstart

ondrag

ondragend

The ondragstart event initiates the drag operation. You can provide a handler for this event to initiate or cancel drag operations selectively. To cancel a drag operation, call the preventDefault method of the event object. To handle an event, assign a value to the effectAllowed property and put the data for the drag in the dataTransfer object, which you can get from the event object. See Changing Drag Effects for information on the effectAllowed property. See Manipulating Dragged Data for information on handling the drag data.

Once a drag is under way, the ondrag event is fired continuously at the element to give it a chance to perform any tasks it wants to while the drag is in progress. Upon completion of the operation, the element receives the ondragend event. If the drag was successful, the ondrop handler for the drop target element is also called (before the ondragend handler is called).

While a drag is in progress, events are sent to elements that are potential drop targets for the contents being dragged. Those elements can handle the following events:

ondragenter

ondragover

ondragleave

ondrop

The ondragenter and ondragleave events let the element know when the user’s mouse enters or leaves the boundaries of the element. You can use these events to change the cursor or provide feedback as to whether a drop can occur on an element. The ondragover event is sent continuously while the mouse is over the element to give it a chance to perform any needed tasks. If the user releases the mouse button, the element receives an ondrop event, which gives it a chance to incorporate the dropped content.

If you implement handlers for the ondragenter and ondragover events, you should call the preventDefault method of the event object. This method takes no parameters and notifies WebKit that your handler will act as the receiver of any incoming data. If you do not call this method, WebKit receives the data and incorporates it for you. You do not need to call preventDefault if you simply want to be notified when the events occur.

Note: You must, at minimum, implement ondragover and call the preventDefault method on the event object. If you do not do this, you will not receive any of these four events.

Adding Handlers to Elements

You can add handlers for drag and drop events to any element in a webpage. When a drag or drop operation occurs, WebKit looks for the appropriate handler on the element that is the focus of the operation. If that element does not define a handler, WebKit walks up the list of parent elements until it finds one that does. If no element defines a handler, WebKit applies the default behavior. To demonstrate this process, suppose you have the following basic HTML in a webpage:

<body ondragstart="BodyDragHandler()"

ondragend="BodyDragEndHandler()">

<span ondragstart="SpanDragHandler()">Drag this text.</span>

</body>

If a user initiates a drag operation on the text in the span tag, WebKit calls SpanDragHandler to handle the event. When the drag operation finishes, WebKit calls the BodyDragEndHandler to handle the event.

Making an Element Draggable

WebKit provides automatic support to let users drag common items, such as images, links, and selected text. You can extend this support to include specific elements on an HTML page. For example, you could mark a particular div or span tag as draggable.

To mark an arbitrary element as draggable, add the -webkit-user-drag attribute (previously -khtml-user-drag) to the style definition of the element. Because it is a cascading style sheet (CSS) attribute, you can include it as part of a style definition or as an inline style attribute on the element tag. The values for this attribute are listed in Table 1.

Table 1 Values for -webkit-user-drag attribute

Value

Description

none

Do not allow this element to be dragged.

element

Allow this element to be dragged.

auto

Use the default logic for determining whether the element should be dragged. (Images, links, and text selections can be dragged but all others cannot.) This is the default value.

The following example shows how you might use this attribute in a span tag to permit the dragging of the entire tag. When the user clicks on the span text, WebKit identifies the span as being draggable and initiates the drag operation.

Manipulating Dragged Data

When an event occurs, your handler uses the dataTransfer object attached to the event to get and set the clipboard data. This object defines the clearData, getData, and setData methods to allow you to clear, get, and set the data on the dragging pasteboard.

Note: For security purposes, the getData method can be called only from within the ondrop event handler.

Unlike many other browsers, the WebKit drag-and-drop implementation supports data types beyond those that are found in HTML documents. When you call either getData or setData, you specify the MIME type of the target data. For types it recognizes, WebKit maps the type to a known pasteboard type. However, you can also specify MIME types that correspond to any custom data formats your application understands. For most drag-and-drop operations, you will probably want to work with simple data types, such as plain text or a list of URIs.

Like applications, WebKit supports the ability to post the same data to the pasteboard in multiple formats. To add another format, you simply call the setData method with a different MIME type and a string of data that conforms to that type.

To get a list of types currently available on the pasteboard, you can use the types property of the dataTransfer object. This property contains an array of strings with the MIME types of the available data.

Changing Drag Effects

When dragging content from one place to another, it might not always make sense to move that content permanently to the destination. You might want to copy the data or create a link between the source and destination documents instead. To handle these situations, you can use the effectAllowed and dropEffect properties of the dataTransfer object to specify how you want data to be handled.

The effectAllowed property tells WebKit what types of operation the source element supports. You would typically set this property in your ondragstart event handler. The value for this property is a string, whose value can be one of those listed in Table 2.

Table 2 Options for dragging and dropping an element

Value

Description

none

No drag operations are allowed on the element.

copy

The contents of the element should be copied to the destination only.

link

The contents of the element should be shared with the drop destination using a link back to the original.

move

The element should be moved to the destination only.

copyLink

The element can be copied or linked.

copyMove

The element can be copied or moved. This is the default value.

linkMove

The element can be linked or moved.

all

The element can be copied, moved, or linked.

The dropEffect property specifies the single operation supported by the drop target (copy, move, link, or none). When an element receives an ondragenter event, you should set the value of this property to one of those values, preferably one that is also listed in the effectAllowed property. If you do not specify a value for this property, WebKit chooses one based on the available operations (as specified in effectAllowed). Copy operations have priority over move operations, which have priority over link operations.

When these properties are set by the source and target elements, WebKit displays feedback to the user about what type of operation will occur if the dragged element is dropped. For example, if the dragged element supports all operations but the drop target only supports copy operations, WebKit displays feedback indicating a copy operation would occur.

Changing the Appearance of Dragged Elements

During a drag operation, WebKit provides feedback to the user by displaying an image of the dragged content under the mouse. The default image used by WebKit is a snapshot of the element being dragged, but you can change this image to suit your needs.

Changing the Snapshot With CSS

The simplest way to change the drag-image appearance is to use cascading style sheet entries for draggable elements. WebKit defines the -webkit-drag (formerly -khtml-drag) pseudoclass, which you can use to modify the style definitions for a particular class during a drag operation. To use this pseudoclass, create a new empty style-sheet class entry with the name of the class you want to modify, followed by a a colon and the string -webkit-drag. In the style definition of this new class, change or add attributes to specify the differences in appearance between the original element and the element while it is being dragged.

The following example shows the style-sheet definition for an element. During normal display, the appearance of the element is determined by the style-sheet definition of the divSrc4 class. When the element is dragged, WebKit changes the background color to match the color specified in the divSrc4:-webkit-drag pseudoclass.

#divSrc4 {

display:inline-block;

margin:6;

position:relative;

top:20px;

width:100px;

height:50px;

background-color:rgb(202,232,255);

}

#divSrc4:-webkit-drag {

background-color:rgb(255,255,154)

}

Specifying a Custom Drag Image

Another way to change the drag image for an element is to specify a custom image. When a drag operation begins, you can use the setDragImage method of the dataTransfer object. This method has the following definition:

function setDragImage(image, x, y)

The image parameter can contain either a JavaScript Image object or another element. If you specify an Image object, WebKit uses that image as the drag image for the element. If you specify an element, WebKit takes a snapshot of the element you specify (including its child elements) and uses that snapshot as the drag image instead.

The x and y parameters of setDragImage specify the point of the image that should be placed directly under the mouse. This value is typically the location of the mouse click that initiated the drag, with respect to the upper-left corner of the element being manipulated.

Unfortunately, obtaining this information in a cross-browser fashion is easier said than done. There is no standard way to determine the position of the mouse relative to the document because different browsers implement the standard event values in subtly incompatible ways.

For the purposes of Safari and WebKit, clientX and clientY are document relative, as are pageX and pageY (which are thus always equal to clientX and clientY).

Complete Example

No description of drag and drop would be complete without a working example. Save this into an HTML file and open it in Safari. You should see a very simple set of boxes containing words. If you drag each word box into the blue “target” box, the box will disappear and the word will appear in its correct place to form the phrase “This is a test”.